1. Field of the Invention
The invention relates in general to the field of logic circuits, and, more particularly, to logic circuits used in computer systems. Specifically, the invention relates to an input buffer to interface between a central processing unit and other components on a main logic circuit within a mixed voltage environment.
2. Description of the Related Art
With the growing complexity of modern computer systems and the availability of various types of peripheral devices, designers are constantly seeking more efficient methods to improve the circuits that serve as interfaces between these peripheral devices and the main circuit board. External devices attached to a computer system are generally referred to as peripheral devices. The main circuit board in a computer system is often referred to as the motherboard. It normally contains the central processing unit (e.g., a microprocessor) and many of the microcontrollers that control standard peripheral devices, such as the display screen, keyboard, and disk drive.
One skilled in the art will appreciate that if a key on a keyboard is depressed, distinct pieces of information (i.e., data) is generated. In most modern computer systems, data is in the form of bits (i.e., binary digits). Bits are typically represented as voltages. When positive logic is used, a bit value of one (i.e., a logic one) is generally represented as a logically high voltage, while a bit value of zero (i.e., a logic zero) is generally represented as a logically low voltage. A logically high voltage, as defined in this application, is a voltage substantially equal to a supply voltage V.sub.cc when used "on chip" (e.g., on the motherboard). A logically low voltage is a voltage level substantially equal to a ground voltage V.sub.ss.
Data is commonly sent from the microprocessor to peripheral devices on the motherboard. Modern computer systems may contain several peripheral devices and other digital devices connected to each other by a bus. The bus transports data among the microprocessors and other components, and is composed of a number of traces. Traces are wire-like connections that are printed on a circuit board and function as transmission lines.
As previously mentioned, data is generally stored as a voltage, and is designated as being in either a logically high or logically low state based on the power supply used to generate that voltage. For example, a logically high voltage on the motherboard would be associated with the reference voltage V.sub.cc, while a logically high voltage generated by the power supply for one of the peripheral devices may be related to a reference voltage V.sub.ccp. Typically, the reference voltage V.sub.ccp from a peripheral power supply is larger than the reference voltage V.sub.cc from the motherboard power supply. The voltage differences between these two logically high levels often warrant an interface circuit between the motherboard and the peripheral devices in a computer system.
Transistors are often considered the building blocks of computing, allowing them to be used to process and store digital signals. Transistors, have an enable terminal that may be used to turn the transistor "on" (i.e., cause it to conduct electricity), depending upon the voltage applied to this terminal. Associated with the turning "on"/"off" of transistors, there is a voltage at which channels form within the transistor and electrical carriers begin to flow more freely. This voltage is generally referred to as the threshold voltage V.sub.t. When voltages above the threshold voltage V.sub.t are applied to the enable terminal of a transistor, it begins to conduct current. Logically low and logically high signals have a range of voltages associated with them. The threshold voltage for a given type of transister (e.g., a PMOS transister) transistor lies within these ranges.
Transistors generally have a maximum voltage difference V.sub.MAXd that can be applied between the enable terminal and either one of the other terminals. When this voltage difference is exceeded, the transistor may become damaged or fail to operate properly. Gate stress generally results when a voltage difference beyond the maximum voltage difference V.sub.MAXd is applied to the enable terminal of a transistor.
To reduce the likelihood of damaging a microprocessor (i.e., central processing unit), signals that are generated by a microprocessor and are to be sent to a peripheral device are generally first sent to an output buffer. The output buffer generally receives the signal and generates a corresponding signal with a voltage level within an allowed range. The output buffer may also be designed to improve the quality of the signal by removing some of the pulse shape distortion such that the signals may be used more effectively by the peripheral devices.
FIG. 1 illustrates a portion of a conventional predriver circuit 100 for an output buffer that may be used as an interface circuit between a peripheral device and a main circuit board. This stage of the predriver circuit 100 receives data from the processor core on a line 103 and generates a corresponding output signal on a line 105 that may be sent to another portion of the output buffer. One skilled in the art will appreciate that a transistor 110, when enabled, may be used to apply a logically high signal to the line 105. The transistor 110 is connected to a peripheral voltage supply 111, which generates a voltage V.sub.ccp.
Similarly, a transistor 115, when enabled, may be used to apply a logically low signal to the line 105 using a connection to a ground voltage supply 116. A parallel pair of transistors 120,125 are serially connected together and cross-coupled with the transistors 110,115. When a logically low signal is applied to the line 103, biasing the transistor 115 on, and pulling the line 105 to ground V.sub.ss. The low voltage on the line 105 biases the transistor 120 on, and applying V.sub.ccp to the gate of the transistor 110, which gate stresses the transistor 110.
The present invention is directed to overcoming or at least reducing one or more of the problems set forth above.